As a charging system for a secondary battery, for example, a lithium ion secondary battery, a constant current constant voltage (CCCV) charging system in which constant current charging and constant voltage charging are combined is known. In the CCCV charging system, charging is performed at a constant current until the battery voltage reaches a predetermined voltage, and charging is performed at a constant voltage after the predetermined voltage is attained. Then, charging is completed at a time point when the charging current converges to substantially 0. As a power supply circuit for charging, a highly-efficient and low-noise resonant converter is used, for example.
For example, as described in Patent Literature 1, a configuration in which a power factor correction converter and a series resonant converter are connected in series is known. The output of the power factor correction converter is fixed at a constant voltage, and charging control is performed by controlling the output of the series resonant converter. The configuration described in Patent Literature 1 is for achieving constant current properties at a low voltage and a low current for preliminary charging. In Patent Literature 1, the output of the power factor correction converter is switched between two levels, and the output voltage is controlled by a DC-DC converter for respective outputs. As a result, a low output voltage can be output.